Pressure gauge chip and manufacturing process thereof

ABSTRACT

The present invention is related to a sensor. In particular, the present invention is related to a pressure sensor die and its fabrication process. The pressure sensor comprises a chamber inside which a pressure sensor die is provided. The pressure sensor die is uniformly compressed by the external pressure to be measured and can deform freely inside the chamber. The pressure sensor die is primarily constructed of single crystalline silicon and comprises a substrate and a cap connected together. A recess is formed on the cap. The recess forms a sealed cavity with the substrate. A silicon oxide layer is formed between the substrate and the cap. The substrate further comprises a plurality of piezoresistive sensing elements which are located inside the sealed cavity. The present pressure sensor is more immune to temperature effects. It is especially suitable for operating in a high temperature, high pressure environment and is capable of delivering accurate and reliable pressure measurements at low cost.

TECHNICAL FIELD

The present invention is related to a sensor. In particular, the presentinvention relates to a pressure sensor for downhole pressuremeasurements.

BACKGROUND OF THE INVENTION

Downhole pressure measurements are essential when drilling forhydrocarbon recovery. During the drilling process, geological pressuredata are collected to tailor drilling parameters and the construction ofthe well. After the well is drilled and production starts, pressure iscontinuously monitored for reservoir management. Accurate measurement ofpressure is therefore the key to optimize recovery and reduce riskthroughout the entire life of a hydrocarbon well. Thus, we need anaccurate and cost-effective pressure sensor for downhole measurements.

Pressure sensors usable in hydrocarbon wells must be able to withstandharsh conditions and remain accurate, stable and reliable for weeksduring a measurement period. In particular, such sensors must be able towithstand temperature ranging from −50° C. to 250° C. and pressure up to200 MPa (around 2000 atmospheres) while maintaining an accuracy ofbetter than 0.1%, and desirably 0.01%, of the full-scale pressure.

Two types of pressure sensors are commonly used for downholeapplications. The first type is the resonant quartz pressure sensor. InU.S. Pat. No. 3,617,780, one example of resonant quartz pressure sensoris described wherein a crystalline quartz cylinder closed at both endsis immersed in a fluid which communicates with the external pressure tobe measured via an isolation diaphragm or a bellow. A crystalline quartzplate spans across the vacuum sealed cavity inside the cylinder. Theplate resonance is excited and detected via the piezoelectric effect.The plate resonant frequency, which varies with the hydrostatic pressureon the cylinder wall, is a measure of the external pressure. Constructedalmost entirely out of crystalline quartz and being a mature technology,resonant quartz pressure sensors have achieved the highest benchmark foraccuracy, stability and reliability for downhole pressure measurementsto date. However, they tend to be very expensive.

The second type of downhole pressure sensors is based on sapphire. InU.S. Pat. No. 5,024,098, a sapphire pressure sensor is described whereina sapphire cell is immersed in a fluid which communicates with theexternal pressure to be measured via an isolation diaphragm. The celldeforms under pressure and the resulting strains are measured by straingauge elements disposed on a planar surface of the sapphire cell. Whilereliable and rugged for downhole applications, sapphire pressure sensorsare in general not as stable and accurate as resonant quartz pressuresensors and they are also quite expensive. In case silicon strain gaugeelements are employed, accuracy and stability could be affected by theexcessive temperature coefficient of resistance and the temperaturecoefficient of piezoresistance effect in silicon. On the other hand, ifnon-silicon strain gauge elements, for example, metallic alloys, areused their low gauge factor and therefore low sensitivity can result inthe undesirable amplification of temperature and other measurementerrors. In any case, the mismatch in the thermal expansion coefficientsbetween sapphire and the strain gauge material creates furthertemperature errors.

The majority of sensors in use today are of the micro-electro-mechanicalsystem (MEMS) type. MEMS based sensors are typically realized withsilicon micromachining that originated from integrated circuitfabrication and still shares many of its processing technologies. Inaddition, there are a few unique processes specifically tailored towardthe fabrication of 3-dimensional microstructures. These includedouble-side photolithography, deep reactive ion etching (DRIE), andwafer bonding to name a few. Silicon has superb mechanical propertiescompared with quartz and sapphire, for example, high hardness, highmodulus of elasticity, high ultimate strength, and is perfectly elasticup to the fracture point. Moreover, single crystalline silicon is highlypiezoresistive, which is therefore effective in converting changes inmechanical strain into changes in electrical resistance. Furthermore,precision microstructures are much easier to fabricate in silicon thanin quartz or sapphire. With demonstrated advantages that include lowcost, small size, high accuracy, high reliability, and high stability,silicon MEMS piezoresistive pressure sensors have become the dominanttype of pressure sensors in use for automotive, medical, industrial andconsumer electronics applications.

Despite their huge success, MEMS pressure sensors have not been widelyadopted for downhole applications. There are a few problems that must beovercome. In particular, an improved mechanical design over theconventional diaphragm-type silicon pressure sensors is required tohandle the very high pressure. This is because in a conventionaldiaphragm-type silicon pressure sensor die, the silicon thin diaphragmserves the purpose of amplifying pressure into stress. In order tomeasure high pressure up to 200 MPa, the lateral dimensions andthickness of the diaphragm must be respectively narrowed down andthickened accordingly. However, if the lateral dimensions of thediaphragm are made too narrow, there will not be enough room on thediaphragm to place the piezoresistive sensing elements. Else if thediaphragm is thickened substantially, it will lead to non-idealdeformation of the entire pressure sensor die. Furthermore, there needsto be a better means in MEMS pressure sensors to overcome the varioustemperature coefficients and instabilities so as to improve themeasurement accuracy at high temperature. Accordingly, a need presentlyexists for an improved silicon pressure sensor that is highly accurate,cost effective, and suitable for operating in a high temperature, highpressure downhole environment.

SUMMARY OF THE INVENTION

The objective of the present invention is to overcome currenttechnological shortcomings so as to provide a pressure sensor that ishighly accurate, having a wide pressure range, less affected by theenvironment, and capable of operating in a high temperature, highpressure downhole environment.

A pressure sensor comprising:

a chamber and a pressure sensor die provided within said chamber, saidpressure sensor die is uniformly compressed by the external pressure tobe measured and can deform freely inside the chamber; said pressuresensor die is constructed of single crystalline silicon and comprises asubstrate and a cap connected together;

wherein a recess is formed on said cap; said recess and said substrateform a sealed cavity; a silicon oxide layer is formed between saidsubstrate and said cap;

said substrate further comprises a plurality of piezoresistive sensingelements; said piezoresistive sensing elements are located inside saidsealed cavity.

The pressure sensor in the present invention also comprises thefollowing optional features:

Said cavity is a vacuum sealed cavity.

A metal contact is provided at the terminals of said piezoresistivesensing element.

Said substrate comprises at least two sets of piezoresistive sensingelements; said two sets of piezoresistive sensing elements areperpendicular to each other.

Said piezoresistive sensing elements are electrically connected in aWheatstone bridge configuration.

Said substrate is formed on a {110} crystallographic plane of p-typesilicon; said piezoresistive sensing elements are formed on n-type dopedregions of said substrate and oriented along a <100> or <110>crystallographic direction.

Said substrate is formed on a {110} crystallographic plane of n-typesilicon; said piezoresistive sensing elements are formed on p-type dopedregions of said substrate and oriented along a <100> or <110>crystallographic direction.

Said cap and said substrate are both cuboids; said cap is formed on a{110} crystallographic plane of single crystalline silicon; and on thebonded plane between said cap and said substrate, the correspondingedges of said cap and said substrate are oriented along the samecrystallographic direction.

Said substrate of said pressure sensor die uses a silicon-on-insulator(SOI) construction; said silicon-on-insulator construction comprises ahandle layer, a device layer, and a buried silicon oxide layer formedbetween said handle layer and device layer; said piezoresistive sensingelements are formed on said device layer.

A silicon oxide insulating layer is formed on the top, the bottom andalong the sides of said piezoresistive sensing element.

Said device layer is formed on a {110} crystallographic plane of p-typesilicon; said piezoresistive sensing elements are formed on said p-typesilicon of said device layer and oriented along a <110> or <110>crystallographic direction.

Said device layer is formed on a {110} crystallographic plane of n-typesilicon; said piezoresistive sensing elements are formed on said n-typesilicon of said device layer and oriented along a <110> or <110>crystallographic direction.

Said handle layer is formed on a {110} crystallographic plane of singlecrystalline silicon; and the corresponding edges of said handle layerand said device layer are oriented along the same crystallographicdirection.

Said cap is formed on a {110} crystallographic plane of singlecrystalline silicon; and the corresponding edges of said cap and saiddevice layer are oriented along the same crystallographic direction.

Said chamber is an enclosure within a metal housing; said chamber isfilled with an electrically insulating fluid; and said pressure sensordie is immersed in said electrically insulating fluid.

A metal diaphragm is further provided; said metal diaphragm seals saidelectrically insulating fluid and said pressure sensor die in saidchamber.

External pressure to be measured is transmitted from said metaldiaphragm to said pressure sensor die.

The first fabrication process for said pressure sensor die comprisingthe following steps:

Step 1, grow or deposit a silicon oxide layer on the top surface and thebottom surface of a single crystalline silicon wafer;

Step 2, using photolithography and ion implantation, dope selectiveregions on the top surface of said single crystalline silicon wafer,thus forming a plurality of piezoresistive sensing elements of theopposite dopant type to said single crystalline silicon wafer;

Step 3, using photolithography and ion implantation, highly dopeselective regions on the top surface of said single crystalline siliconwafer, thus forming highly conductive regions of the opposite dopanttype to said single crystalline silicon wafer;

Step 4, using photolithography and ion implantation, highly dopeselective regions on the top surface of said single crystalline siliconwafer, thus forming highly conductive regions of the same dopant type assaid single crystalline silicon wafer; afterward grow or deposit asilicon oxide layer on the top surface and the bottom surface of saidsingle crystalline silicon wafer and activate all said implanted dopantspecies;

Step 5, using photolithography and etching, etch contact holes throughsaid silicon oxide layer over said highly conductive regions reachingsaid highly doped regions; then use metal deposition to form metalinterconnection patterns from said contact holes;

Step 6, bond a single crystalline silicon cap wafer which wasprefabricated with recesses to the top of said single crystallinesilicon wafer;

Step 7, using wafer dicing, cut the bonded silicon wafer into individualpressure sensor dice.

The second fabrication process for said pressure sensor die comprisingthe following steps:

Step 1, grow or deposit a silicon oxide layer on the top surface and thebottom surface of a silicon-on-insulator wafer;

Step 2, using photolithography and ion implantation, highly dopeselective regions on the device layer of said silicon-on-insulatorwafer, thus forming highly conductive regions of the same dopant type assaid device layer;

Step 3, using photolithography and etching, etch trenches through saiddevice layer reaching said buried silicon oxide layer to form saidpiezoresistive sensing elements;

Step 4, grow or deposit a layer of silicon oxide to fill said trenchesand activate said implanted dopant species;

Step 5, using photolithography and etching, etch contact holes throughsaid silicon oxide layer over said highly conductive regions reachingsaid highly doped regions on said device layer; then use metaldeposition to form metal interconnection patterns from said contactholes;

Step 6, bond a single crystalline silicon cap wafer which wasprefabricated with recesses to the top of said silicon-on-insulatorwafer;

Step 8, using wafer dicing, cut the bonded silicon wafer into individualpressure sensor dice.

The fabrication process for said recesses on said silicon cap wafercomprises photolithography and etching.

Said etching method comprises one kind or a combination of dry and wetetching methods; said dry etching method is selected from one or more ofthe following methods: deep reactive ion etching, reactive ion etching,or gaseous xenon difluoride etching for silicon; as well as reactive ionetching, plasma etching, or hydrofluoric acid vapor etching for siliconoxide.

Said wet etching method for silicon comprises one kind or a combinationof the following etchants: potassium hydroxide, tetramethylammoniumhydroxide, or ethylenediamine pyrocatechol.

Said wet etching method for silicon oxide comprises one kind or acombination of the following etchants: hydrofluoric acid or bufferedhydrofluoric acid.

Comparing with the two types of downhole pressure sensors mentioned inthe prior art, the pressure sensor in the present invention has thefollowing advantages. First of all, the manufacturing cost of a siliconpressure sensor is much lower than that for quartz and sapphire pressuresensors. However, conventional diaphragm-type silicon pressure sensorscannot function in a 200 MPa environment. In contrast, the pressuresensor die in the present invention does not utilize a diaphragm elementas in conventional silicon pressure sensors. Instead, the sensor die isacted upon on all of its surfaces (the top, the bottom and the foursides) by the high pressure in the downhole. The resulting internal diestress is directly sensed without the need for mechanical amplificationby a silicon diaphragm. This way the present invention overcomes themain difficulty in the mechanical design of diaphragm-type siliconpressure sensors for high pressure applications while retaining theadvantages of silicon MEMS pressure sensors.

Secondly, conventional MEMS piezoresistive sensing elements areelectrically insulated by reverse biased PN junctions, the leakagecurrent of which increases exponentially with temperature. As thetemperature rises above 150° C., the insulation property of the PNjunction will fail. On the other hand, in one of the preferredembodiments of the present invention, there is a buried silicon oxidelayer between the piezoresistive sensing elements and the handle layer.There is also silicon oxide layer in between each piezoresistive sensingelement. Moreover, an oxide layer is grown or deposited on top of thepiezoresistive sensing elements. As a result, each piezoresistivesensing element is completely wrapped around by silicon oxideinsulation. Using this dielectric isolation scheme, the electricalinsulation will operate even at high temperature.

Furthermore, in the present invention, the two perpendicular sets ofpiezoresistive sensing elements on the pressure sensor die havedifferent pressure responses, hence providing a differential change inelectrical resistance. Then by connecting all four piezoresistivesensing elements in a Wheatstone bridge configuration, temperature andother common mode errors are significantly reduced, thereby increasingthe accuracy of the present pressure sensor.

Additionally, the internal cavity formed between the substrate and thecap is preferably sealed in vacuum. The critical portions of all thepiezoresistive sensing elements are located inside this vacuum sealedcavity where these critical portions of the piezoresistive sensingelements are least susceptible to external interferences, such as localtemperature fluctuations, and foreign contaminations, such as dust. Thereliability of the present pressure sensor is further improved as aresult. Lastly, the entire pressure sensor die primarily uses a siliconconstruction which not only avoids the problems caused by the mismatchbetween dislike materials, but also enables the use of MEMS fabricationtechnologies with much lower manufacturing cost than that for the quartzand sapphire pressure sensors in the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the pressure sensor die in the firstembodiment of this invention.

FIG. 2 is a perspective view of the pressure sensor die of FIG. 1further with the cap detached revealing a recess underneath and furtherwith the top silicon oxide layer removed revealing various features onthe substrate in the first embodiment of this invention.

FIG. 3 is a plan view of the pressure sensor die in the first embodimentof this invention.

FIG. 4 is a perspective view of the pressure sensor die in the secondembodiment of this invention.

FIG. 5 is a perspective view of the pressure sensor die of FIG. 4further with the cap detached revealing a recess underneath and furtherwith the top silicon oxide layer removed revealing various features onthe device layer in the second embodiment of this invention.

FIG. 6 is a cutaway view of the pressure sensor die of FIG. 5 along lineAA′.

FIG. 7 is a plan view of the pressure sensor die in the secondembodiment of this invention.

FIG. 8 is a circuit diagram of the piezoresistive sensing elementsconnected in a Wheatstone bridge configuration.

FIG. 9 is a diagrammatic view of the pressure sensor.

FIG. 10A is the variation of the piezoresistive coefficients π₁₁+π₁₂versus crystallographic orientation on a {110} crystallographic plane ofp-type silicon.

FIG. 10B is the variation of the piezoresistive coefficients π₁₁+π₁₂versus crystallographic orientation on a {110} crystallographic plane ofn-type silicon.

FIG. 11 is a cross-sectional view illustrating step 1 and step 2 of thefabrication process in the first embodiment of the pressure sensor die.

FIG. 12 is a cross-sectional view illustrating step 3 and step 4 of thefabrication process in the first embodiment of the pressure sensor die.

FIG. 13 is a cross-sectional view illustrating step 5 and step 6 of thefabrication process in the first embodiment of the pressure sensor die.

FIG. 14 is a cross-sectional view illustrating step 7 of the fabricationprocess in the first embodiment of the pressure sensor die.

FIG. 15 is a cross-sectional view illustrating step 1 and step 2 of thefabrication process in the second embodiment of the pressure sensor die.

FIG. 16 is a cross-sectional view illustrating step 3 and step 4 of thefabrication process in the second embodiment of the pressure sensor die.

FIG. 17 is a cross-sectional view illustrating step 5 and step 6 of thefabrication process in the second embodiment of the pressure sensor die.

FIG. 18 is a cross-sectional view illustrating step 7 of the fabricationprocess in the second embodiment of the pressure sensor die.

DETAILED DESCRIPTION

The illustrative embodiments of the present invention will be describedin detail with reference to the accompanying drawings. Please note thatthe scope of the present invention is not limited to these preciseembodiments described. Various changes or modifications may be effectedtherein by one skilled in the art without departing from the scope orspirit of the invention.

With reference to FIGS. 1-3, a pressure sensor die is shown according tothe first embodiment of the present invention. The pressure sensor dieis primarily constructed of single crystalline silicon and comprises asubstrate 6 and a cap 3 connected together. Using a silicon constructionhelps reduce the measurement errors caused by the mismatch in thermalexpansion coefficients between dissimilar materials. A recess 5 isformed on the cap 3, which forms a sealed cavity after bonding with thesubstrate 6. In the plane view shown in FIG. 3, the bonded plane betweenthe cap and the substrate is outlined by the dashed lines. The substratefurther comprises a plurality of piezoresistive sensing elements 23, thecritical portions of the piezoresistive sensing elements 23 are locatedinside the sealed cavity. Preferably, the sealed cavity is a vacuumsealed cavity; this further reduces the undesirable effects of foreigncontaminants and local temperature fluctuations on the piezoresistivesensing elements 23. A silicon oxide layer 4 is formed between cap 3 andsubstrate 6. Metal contacts 8 are provided on top of silicon oxide layer4. These metal contacts 8 are respectively connected to the terminals ofpiezoresistive sensing elements 23. External electrical circuits andcomponents are only connected to metal contacts 8; thus it furtherreduces the effects of undesirable interferences on piezoresistivesensing elements 23.

With reference to FIG. 3, in this first embodiment of the presentinvention, four identical piezoresistive sensing elements 23 areprovided on the substrate 6. These are R1 to R4 in which R1, R3 and R2,R4 are perpendicular to each other. Each piezoresistive sensing elementemploys a basic U-shaped design. Preferably, the piezoresistive sensingelement 23 comprises a few U-shaped segments connected together to forma serpentine structure. In the first embodiment, piezoresistive sensingelements 23 are diffused resistors formed by doping selective regions onsubstrate 6. They are electrically insulated from one other via reversedbiased PN junctions. The dopant type for piezoresistive sensing elements23 is according to whether substrate 6 is p-type or n-type. For p-typesubstrate 6, an n-type dopant is used. For n-type substrate 6, a p-typedopant is used instead. Preferably, each piezoresistive sensing element23 is further formed with type-A highly doped regions 9, the purpose ofwhich is to increase the doping level in selective portions ofpiezoresistive sensing element 23. Therefore, the dopant type for thetype-A highly doped regions 9 must be the same as that in the originalnon-highly doped portions of piezoresistive sensing element 23. Sincethe sheet resistance in the non-highly-doped portion is approximately100 Ω/square whereas the sheet resistance in the highly-doped portion isonly 15 Ω/square, having type-A highly doped regions 9 in eachpiezoresistive sensing element 23 locally reduces the electricalresistance there, thus forming highly conductive regions. As a result,the total electrical resistance in each piezoresistive sensing element23 primarily comes from a few remaining longitudinal segments that arenon-highly doped. In actual pressure measurements, it is thesenon-highly doped longitudinal segments, which are housed inside thesealed cavity and oriented in the same direction, that produce thelargest electrical resistance change. Reducing the electrical resistanceof piezoresistive sensing elements 23 in selective regions using type-Ahighly doped regions 9 therefore increases the overall percentage changein the electrical resistance of piezoresistive sensing element 23; andhence improves the measurement accuracy of the pressure sensor.Moreover, through the connections between the metal contacts 8 and thetype-A highly doped regions 9, good electrical contacts between metalcontacts 8 and piezoresistive sensing elements 23 can be assured.Additionally, type-B highly doped regions 10 and their associated metalcontacts 8 are further provided, the purpose of which is for makingelectrical connections between external electrical circuit and theremaining regions of substrate 6 that are outside and therefore do notconstitute a part of piezoresistive sensing elements 23. Theseelectrical connections provide the necessary reverse bias for the PNjunctions. The dopant type for these type-B highly doped regions musttherefore be the same as that in substrate 6 to ensure that the metalcontacts 8 makes good electrical contacts with substrate 6.

Since the four piezoresistive sensing elements R1 to R4 are identical,when the pressure sensor die is not subjected to an external pressure,the electrical resistance in R1 to R4 should be the same in theory.Preferably, the two perpendicular sets of piezoresistive sensingelements, R1, R3 and R2, R4, are oriented along differentcrystallographic directions such that when the pressure sensor die isuniformed compressed and free to deform under the external pressure, thedifference in the piezoresistance effect between the two sets ofpiezoresistive sensing elements, R1, R3 and R2, R4, is maximized, thusresulting in unequal electrical resistance changes. Stress, however, isnot the only factor that affects the electrical resistance values inpiezoresistive sensing elements 23. Other factors, such as thetemperature in the environment, will change the electrical resistancevalues as well. For this, preferably and with reference to FIG. 8,piezoresistive sensing elements R1 to R4 are electrically connected in aWheatstone bridge configuration. A constant current source 24 suppliesthe electric current going through the Wheatstone bridge. The externalpressure can be calculated by measuring the voltage difference betweenpoints V+ and V−. When the pressure sensor die is not subjected toexternal pressure, the electrical resistances in piezoresistive sensingelements R1 to R4 are almost identical; the voltage between points V+and V− is then close to zero. On the other hand, when an externalpressure induces unequal electrical resistance changes between R1, R3and R2, R4, a voltage develops between points V+ and V−. A majoradvantage of the Wheatstone bridge configuration is the reduction ofcommon-mode errors. For example, as the temperature changes, theelectrical resistances of all four piezoresistive sensing elementschange by about the same amount. As a result, when there is no externalpressure, the voltage between points V+ and V− remains close to zero. Inthe present invention, the Wheatstone bridge can be powered by aconstant voltage source or a constant current source, but a constantcurrent source is preferred because the negative temperature coefficientof silicon piezoresistance effect is partially offset by the positivetemperature coefficient of resistance, resulting in an overall reducedscale factor error over temperature. Furthermore, the bridge voltage,represented by Vb which can be measured, contains temperatureinformation which is useful for further temperature error correction.

In comparison with existing technologies in which the piezoresistivesensing elements are all oriented along the same direction, thepiezoresistive sensing elements in the present invention are orientedalong two perpendicular directions. Therefore in pressure measurements,the pressure information collected by the piezoresistive sensingelements is more complete in the present invention. For example, inconventional silicon MEMS diaphragm-type pressure sensors, the pressuresignal is detected by two sets of piezoresistive sensing elementsoriented along the same direction, where each set contains twopiezoresistive sensing elements. Due to geometric constraints at thediaphragm edge, the shape and layout of the two sets of piezoresistivesensing elements often cannot be made identical. This can give rise to amismatch in the electrical resistance values or in the temperaturecoefficients of resistance between the two sets, resulting in anincomplete cancellation of common mode errors after processing throughthe Wheatstone bridge. Although this residual error can be furthercorrected via analog or digital compensation, some pressure accuracy isinevitably sacrificed. In contrast, the pressure signal in the presentinvention is derived from the differential output of two sets ofpiezoresistive sensing elements which are identical in shape and layout.Hence the pressure accuracy can be higher.

With reference to FIGS. 1 and 3, the preferable size of the presentpressure sensor die is approximately 1.6 mm in length, 1.6 mm in widthand 1 mm in thickness. The internal sealed cavity measures approximately0.4 mm in length, 0.4 mm in width and 0.2 mm in height. Inmanufacturing, an 8 inch silicon wafer can contain thousands to over10,000 gross pressure sensor dice, thus resulting in a significantreduction in die cost. However, it should be noted that the foregoingdimensions of the preferred embodiment are for illustrative purposesonly. The present invention is not limited to this embodiment and alldimensions can be tailored for a particular design.

In the first embodiment, the piezoresistive sensing elements 23 areelectrically insulated by reverse biased PN junctions, the leakagecurrent of which increases exponentially with temperature. As thetemperature rises above 150° C., the insulating property of the reversebiased PN junction will fail. Therefore the first embodiment is onlysuitable for applications in which the temperature is below 150° C.

With reference to FIGS. 4-7, a pressure sensor die is shown according tothe second embodiment of the present invention. The operating principle,die size and external electrical circuit in this embodiment are the sameas those in the first embodiment with the exception that the substrate 6is using an SOI construction which comprises a handle layer 1, a devicelayer 2, and a silicon oxide layer 4 between handle layer 1 and devicelayer 2 all connected together. The silicon oxide layer 4 is alsoreferred to as a “buried” silicon oxide layer and serves to be theelectrical insulation between handle layer 1 and device layer 2.Distinct from the first embodiment, the piezoresistive sensing elements23 in the second embodiment are formed on device layer 2. Moreover, asilicon oxide layer 4 is formed along the sidewalls of eachpiezoresistive sensing element 23. As a result, each piezoresistivesensing element is completely wrapped around and fully insulated with alayer of silicon oxide 4 on the top, the bottom and along the sides. Notonly does this dielectric isolation scheme reduce crosstalk andinterference among sensing elements, it also enables the pressure sensordie to operate at temperature as high as 250° C. and not limited by thePN junction insulation failure as in the first embodiment. Furthermore,type-A highly doped regions 9 are formed at the two terminals as well asat the turnaround corners of each piezoresistive sensing element 23, thepurpose of which is to increase the doping level in selective portionsof piezoresistive sensing element 23, thus forming high conductiveregions, the function of which is the same as in the first embodiment.However, the piezoresistive sensing elements 23 in the second embodimentare single crystalline silicon resistors formed on device layer 2, thedopant type for the type-A highly doped regions 9 must therefore be thesame as that in device layer 2. In this embodiment, the thickness ofdevice layer 2 is approximately 2 μm and the thickness of the buriedoxide layer is approximately 1 μm. However, it should be noted that theforegoing dimensions of the preferred embodiment are for illustrativepurposes only. The present invention is not limited to this embodimentand all dimensions can be tailored for a particular design.

FIG. 9 illustrates a diagrammatic view of the pressure sensor, which canbe applied to the first and the second embodiments of the pressuresensor die. Pressure sensor die 31 is installed within chamber 33, whichis an enclosure defined within a metal housing. Chamber 33 is filledwith electrical insulating fluid 37 in which pressure sensor die 31 isimmersed. In one embodiment, metal diaphragm 38 is provided to seal bothpressure sensor die 31 and electrical insulating fluid 37 within chamber33. External pressure 39 acting on metal diaphragm 38 is transmitted viaelectrical insulating fluid 37 to the pressure sensor die 31.Preferably, metal diaphragm 38 is a corrugated baffle with a lowstiffness; so almost all of external pressure 39 is transmitted topressure sensor die 31. Furthermore, the contact between pressure sensordie 31 and the chamber housing is kept to a minimum. For example,pressure sensor die 31 is only attached to the chamber housing throughone or several dots or lines of die adhesive 32. Moreover, die adhesive32 is compliant enough so that under the action of the hydrostaticpressure in electrical insulating fluid 37, pressure sensor die 31 isuniformly compressed and free to deform. Such installation furtherprevents packaging stresses, which can be caused by the deformation ofthe chamber housing due to external forces or temperature change, frombeing transmitted to pressure sensor die 31.

Preferably, said cavity within the pressure sensor die 31 formed betweensubstrate 6 and cap 3 is vacuum sealed, so that the pressure measured bythe pressure sensor is absolute pressure referenced to vacuum. In oneembodiment, metal bond pads on pressure sensor die 31 are connected viabond wires 34 to metal pillars 35, which are in turn connected to theexternal electrical circuit. Metal pillars 35 are electrically insulatedfrom one another by insulator 36.

Next the crystallographic orientations of the pressure sensor die 31 andpiezoresistive sensing elements 23 will be described. Regarding thesilicon piezoresistance effect, besides varying with stress, the siliconelectrical resistivity further varies with the dopant type (p or n),doping concentration, and crystallographic orientation since singlecrystalline silicon is anisotropic, the details for which are describedin Y. Kanda, “A Graphical Representation of the PiezoresistanceCoefficients in Silicon,” IEEE Transactions on Electron Devices, vol.ED-29, no. 1, pp. 64-70, 1982. In particular, the change in electricalresistivity and its relationship with stresses and piezoresistivecoefficients can be expressed asΔρ₁₁/ρ=π₁₁σ₁₁+π₁₂σ₂₂+π₁₃σ₃₃+π₁₄σ₂₃+π₁₅σ₁₃+π₁₆σ₁₂  (1)

where 1, 2, 3 are the three orthogonal directions in a Cartesiancoordinate system; Δρ₁₁/ρ is the relative change in silicon resistivitywhen both the electric field and electric current are along direction 1;σ₁₁, σ₂₂, σ₃₃ are the respective normal stresses along the 1, 2, 3directions; σ₂₃, σ₁₃, σ₁₂ are the respective shear stresses along the2-3, 1-3, 1-2 directions; and π₁₁, π₁₂, π₁₃, π₁₄, π₁₅, π₁₆ are thepiezoresistive coefficients expressing the relationship betweenresistivity change and the various stresses. Assume that the top surfaceof substrate 6 of pressure sensor die 31 is perpendicular to direction 3on plane 1-2, and that the piezoresistive sensing elements 23 arelocated on this plane. With reference to FIGS. 2 and 5, first of all,the critical (non-highly doped) portions of the four piezoresistivesensing elements 23 are facing a vacuum inside the sealed cavity. Theplane 1-2 on which they are located is therefore a free surface. As aresult, the stress components associated with the normal direction 3,i.e., σ₁₃, σ₂₃, σ₃₃, are all zero. Secondly, pressure sensor die 31 isinstalled inside chamber 33. The sensor die is uniformly compressed, andit is allowed to deform freely. Under these circumstances, the normalstresses acting on the external surfaces of the pressure sensor die areall comparable to the external pressure 39, and all the shear stressesare close to zero. Then further into the vacuum sealed cavity, on thesaid plane 1-2 on which piezoresistive sensing elements 23 are located,σ₁₂ is still close to zero, whereas σ₁₁ and σ₂₂ are comparable to eachother and both scale approximately linearly with the external pressure39 P. The magnitudes of σ₁₁ and σ₂₂ are somewhat smaller than P though.Preferably, the thickness of substrate 6 is larger than the cavitywidth. Otherwise, under the action of external pressure 39, substrate 6will bulge toward the sealed cavity. This will lead to even smallermagnitudes of σ₁₁ and σ₂₂ in comparison with P. The sensitivity of thepressure sensor die will be reduced as a result but still usable.Summarizing all of the above, when applied to the critical (non-highlydoped) portions of piezoresistive sensing elements 23, Equation (1) canbe approximated by this simplified expressionΔρ₁₁/ρ≈(π₁₁+π₁₂)k P  (2)

where k is a constant less than 1.

Since electrical resistance is proportional to resistivity, fromEquation (2), the electrical resistance changes in piezoresistivesensing elements 23 scale approximately linearly with the externalpressure 39, whereas the pressure sensitivity is directly proportionalto the piezoresistive coefficients π₁₁+π₁₂. However, since singlecrystalline silicon is anisotropic, π₁₁+π₁₂ will vary according to theorientations of plane 1-2 and direction 1 as well. For example, if plane1-2 is a {110} crystallographic plane of silicon, with reference to FIG.10, when direction 1 rotates from 0° to 360°, the piezoresistivecoefficients π₁₁+π₁₂ will vary as shown in FIG. 10. Therefore on the{110} crystallographic plane of silicon, whether it is p-type or n-typesilicon, π₁₁+π₁₂ will reach a maximum along the <110> direction andreach a minimum along the <100> direction orthogonal to <110>.

As described in the first and second embodiments of the pressure sensordie 31, preferably, the two sets of piezoresistive sensing elements R1,R3 and R2, R4 can be installed along different crystallographicorientations so that when the pressure sensor die 31 is uniformlycompressed and deforms freely under the external pressure 39, thedifference in piezoresistance effect between the two sets ofpiezoresistive sensing elements R1, R3 and R2, R4, i.e., the differencein the piezoresistive coefficients π₁₁+π₁₂ is maximized. This way, theelectrical resistance change will be different and a voltage output willappear at the Wheatstone bridge. Preferably, the substrate 6 in thefirst embodiment and the device layer 2 in the second embodiment areformed on a {110} crystallographic plane of single crystalline silicon,and the two sets of piezoresistive sensing elements are respectivelyoriented along the orthogonal <100> and <110> crystallographicdirections. With reference to FIGS. 3 and 7, the non-highly dopedportions of piezoresistive sensing elements R1 and R3 are both orientedalong the <100> direction. Therefore their electrical resistance changesare the same. Likewise, the non-highly doped portions of piezoresistivesensing elements R2 and R4 are both oriented along the <110> direction.Therefore their electrical resistance changes are the same as well butdiffer by the maximum amount from the electrical resistance changes ofR1 and R3. Preferably, since the deformation under stress of singlecrystalline silicon is anisotropic, the cap 3 is also formed on a {110}crystallographic plane of single crystalline silicon, so that thepressure sensor die 31 is uniformly compressed. Moreover, on the bondedplane between cap 3 and substrate 6, the corresponding edges on cap 3and substrate 6 are oriented along the same crystallographic direction;or the corresponding edges on cap 3 and handle layer 1 and device layer2 are oriented along the same crystallographic direction. Othercrystallographic orientations of substrate 6 and piezoresistive sensingelements 23 are also feasible, e.g., by referring to Y. Kanda'sdescription.

Next, the first fabrication process for the pressure sensor die isdescribed with reference to FIGS. 11 to 14. This fabrication techniquecan be applied to the first embodiment of the pressure sensor die. Asdescribed above, the starting material for substrate 6 is a singlecrystalline silicon wafer, followed by additional process steps asdescribed in below.

Step 1, form a layer of silicon oxide 4 on the top surface and thebottom surface of the single crystalline silicon wafer by means ofthermal oxidation or chemical vapor deposition method.

Step 2, using photolithography, first coat a layer of photoresist on thetop surface of the single crystalline silicon wafer. Then expose thephotoresist according to certain mask pattern. The exposed photoresistis then dissolved away with a developer, leaving the unexposedphotoresist which is subsequently hard baked. This way the mask patternis transferred onto the photoresist on the top silicon oxide layer 4.Then using ion implantation, the exposed areas on the top silicon oxidelayer 4 is implanted with a dopant ion with sufficient energy topenetrate the silicon oxide layer 4 reaching substrate 6. Meanwhile, theions are stopped by the hard-baked photoresist in the unexposed areasand will not reach substrate 6. This way, selective regions on substrate6 are implanted, forming piezoresistive sensing elements 23 with adopant species of the opposite type to substrate 6. If the substrate 6is of p-type, then an n-type dopant, such as phosphorus ion, can beused. If the substrate 6 is of n-type, then a p-type dopant, such asboron ion, can be used. Lastly, the photoresist is removed. In additionto the ion implantation method, the dopant can also be introduced by ahigh temperature diffusion technique.

Step 3, using photolithography and ion implantation, form type-A highlydoped regions 9 on the top surface of the single crystalline siliconwafer with a dopant species of the same type as piezoresistive sensingelements 23, thus forming highly conductive regions in which theelectrical resistance is greatly reduced. If the substrate 6 is ofp-type, then an n-type dopant, such as phosphorus ion, can be used. Ifthe substrate 6 is of n-type, then a p-type dopant, such as boron ion,can be used.

Step 4, using photolithography and ion implantation, form type-B highlydoped regions on the top surface of the single crystalline silicon waferwith a dopant species of the same type as substrate 6, thus forminghighly conductive regions in which the electrical resistance is greatlyreduced. If the substrate 6 is of p-type, then a p-type dopant, such asboron ion, can be used. If the substrate 6 is of n-type, then an n-typedopant, such as phosphorus ion, can be used. Afterward grow or deposit asilicon oxide layer 4 on the top surface and the bottom surface of thesingle crystalline silicon wafer and activate all the implanted dopantspecies.

Step 5, using photolithography followed by dry or wet etching, etchcontact holes 8 through the silicon oxide layer 4 on top of the singlecrystalline wafer reaching type-A and type-B highly doped regions onsubstrate 6; using metal deposition, photolithography and etching, formmetal interconnection patterns from contact holes 8 to peripheral bondpads.

Step 6, bond a single crystalline silicon cap wafer which wasprefabricated with recesses 5 to the top of the processed singlecrystalline silicon wafer in vacuum to form the vacuum sealed cavity.The bonding technique includes silicon fusion bonding, eutectic bonding,solder bonding, and anodic bonding.

Step 7, using wafer dicing, cut the bonded silicon wafer into individualpressure sensor dice.

Next, the second fabrication process for the pressure sensor die isdescribed with reference to FIGS. 15 to 18. This fabrication techniquecan be applied to the second embodiment of the pressure sensor die. Asdescribed above, the starting material is an SOI wafer that comprises ahandle layer 1, device layer 2, and a buried silicon oxide layer 4formed between the handle layer and the device layer, followed byadditional process steps as described in below.

Step 1, form a layer of silicon oxide 4 on the top surface and thebottom surface of the SOI wafer by means of thermal oxidation orchemical vapor deposition method.

Step 2, using photolithography, first coat a layer of photoresist on thetop surface of the SOI wafer. Then expose the photoresist according tocertain mask pattern. The exposed photoresist is then dissolved awaywith a developer, leaving the unexposed photoresist which issubsequently hard baked. This way the mask pattern is transferred ontothe photoresist on the top silicon oxide layer 4. Then using ionimplantation, the exposed areas on the top silicon oxide layer 4 isimplanted with a dopant ion with sufficient energy to penetrate thesilicon oxide layer 4 reaching the device layer 2. Meanwhile, the ionsare stopped by the hard-baked photoresist in the unexposed areas andwill not reach the device layer 2. This way, selective regions on devicelayer 2 are implanted, forming type-A highly doped regions 9 with adopant species of the same type as device layer 2, where the electricalresistance is greatly reduced thus forming highly conductive regions. Ifthe device layer 2 is of p-type, then a p-type dopant, such as boronion, can be used. If the device layer 2 is of n-type, then an n-typedopant, such as phosphorus ion, can be used. Lastly, the photoresist isremoved. In addition to the ion implantation method, the dopant can alsobe introduced by a high temperature diffusion technique.

Step 3, using photolithography, transfer a mask pattern onto a layer ofphotoresist on the top surface of the SOI wafer. Then etch the topsilicon oxide layer 4 using dry or wet etching to form trenches 11reaching down to device layer 2. Afterward further etch trenches 11 fromdevice layer 2 down to buried silicon oxide layer 4 using deep reactiveion etching or other dry or wet etching methods to form piezoresistivesensing elements 23.

Step 4, use thermal oxidation or chemical vapor deposition method toform a silicon oxide layer 4 that fills trenches 11 and in so doingactivate all implanted dopant species. As a result, the piezoresistivesensing elements 23 are completely wrapped around by a layer of siliconoxide insulation.

Step 5, using photolithography followed by dry or wet etching, etchcontact holes 8 through the top silicon oxide layer 4 on top of thehighly conductive regions 9 reaching the type-A highly doped regions ondevice layer 2; using metal deposition, photolithography and etching,form metal interconnection patterns from contact holes 8 to peripheralbond pads.

Step 6, bond a single crystalline silicon cap wafer which wasprefabricated with recesses 5 to the top of the processed SOI wafer invacuum to form the vacuum sealed cavity. The bonding technique includessilicon fusion bonding, eutectic bonding, solder bonding, and anodicbonding.

Step 7, using wafer dicing, cut the bonded silicon wafer into individualpressure sensor dice.

The etching methods are selected from one or more of the followingmethods: dry etching or wet etching; the dry etching for siliconcomprises deep reactive ion etching, reactive ion etching, and gaseousxenon difluoride etching; and the dry etching for silicon oxidecomprises reactive ion etching, plasma etching, and hydrofluoric acidvapor etching.

The wet etching of silicon comprises one kind or a combination of thefollowing etchants: potassium hydroxide, tetramethylammonium hydroxideor ethylenediamine pyrocatechol.

The wet etching of silicon oxide comprises one kind or a combination ofthe following etchants: hydrofluoric acid or buffered hydrofluoric acid.

The pressure sensor die in the present invention utilizes anon-diaphragm-type novel structure. The sensor die is uniformlycompressed by the external pressure, and the resulting internal stressesare converted via the silicon piezoresistance effect into electricalresistance changes in the piezoresistive sensing elements 23. Theanisotropy of silicon piezoresistance is further exploited for theoptimal placement of two sets of piezoresistive sensing elements 23 onthe same crystallographic plane but along two different crystallographicorientations such that the difference in the electrical resistancechanges between the two sets is maximized, thus enabling the measurementof pressure up to 200 MPa. In addition, the critical portions ofpiezoresistive sensing elements 23 are placed inside a vacuum sealedcavity. This reduces the undesirable influence from the externalenvironment and foreign materials, and increases the reliability andaccuracy of the pressure sensor. Moreover, each piezoresistive sensingelement 23 is completely wrapped around and isolated by a layer ofsilicon oxide insulator 4. This reduces crosstalk and interference amongpiezoresistive sensing elements 23. Such dielectric isolation schemealso enables the present pressure sensor to operate at high temperature.Furthermore, connecting the piezoresistive sensing elements in aWheatstone bridge configuration is the key to reduce common-mode errorsand temperature effects. Finally, manufacturing the pressure sensor dieon a silicon wafer using microfabrication techniques significantlyreduces the manufacturing cost of the pressure sensor die. As describedabove, a single 8-inch silicon wafer can produce thousands to over10,000 pressure sensor dice.

Lastly, it will be appreciated by those of ordinary skill in the artthat many variations in the foregoing preferred embodiments are possiblewhile remaining within the scope of the present invention. The presentinvention should thus not be considered limited to the preferredembodiments or the specific choices of materials, configurations,dimensions, applications or ranges of parameters employed therein.

What we claim in our invention:
 1. A pressure sensor comprising: achamber and a pressure sensor die provided within said chamber; whereinsaid chamber is filled with an electrically insulating fluid; and saidpressure sensor die is immersed in said electrically insulating fluid;said pressure sensor die is attached to said chamber through at leastone dot or line of die adhesive, resulting in said pressure sensor diebeing uniformly compressed by pressure and free to deform; said pressuresensor die is constructed of single crystalline silicon and includes asubstrate and a cap connected together; wherein a recess is formed onsaid cap; said recess and said substrate form a sealed cavity; a siliconoxide layer is formed between said substrate and said cap; saidsubstrate includes at least two sets of piezoresistive sensing elements;said piezoresistive sensing elements are located inside said sealedcavity; said two sets of piezoresistive sensing elements areperpendicular to each other, with each set of piezoresistive sensingelements oriented in a different crystallographic direction.
 2. Thepressure sensor according to claim 1, wherein said sealed cavity is avacuum sealed cavity.
 3. The pressure sensor according to claim 1,wherein a metal contact is provided at the terminals of each of saidpiezoresistive sensing elements.
 4. The pressure sensor according toclaim 1, wherein each of said piezoresistive sensing elements comprisesa plurality of connected U-shaped segments.
 5. The pressure sensoraccording to claim 1, wherein said piezoresistive sensing elements areelectrically connected in a Wheatstone bridge configuration.
 6. Thepressure sensor according to claim 1, wherein said substrate is formedon a {110} crystallographic plane of p-type silicon; said piezoresistivesensing elements are formed on n- type doped regions of said substrate;one set of said piezoresistive sensing elements is oriented along a<100> crystallographic direction, and the other set of saidpiezoresistive sensing elements is oriented along a <110>crystallographic direction.
 7. The pressure sensor according to claim 1,wherein said substrate is formed on a {110} crystallographic plane ofn-type silicon; said piezoresistive sensing elements are formed onp-type doped regions of said substrate; one set of said piezoresistivesensing elements is oriented along a <100> crystallographic direction,and the other set of said piezoresistive sensing elements is orientedalong a <110> crystallographic direction.
 8. The pressure sensoraccording to claim 6, wherein said cap and said substrate are bothcuboids; said cap is formed on a {110} crystallographic plane of singlecrystalline silicon; and on the bonded plane between said cap and saidsubstrate, the corresponding edges of said cap and said substrate areoriented along the same crystallographic direction.
 9. The pressuresensor according to claim 1, wherein said substrate of said pressuresensor die uses a silicon-on-insulator construction; saidsilicon-on-insulator construction includes a handle layer, a devicelayer, and a buried silicon oxide layer formed between said handle layerand device layer; said piezoresistive sensing elements are formed onsaid device layer.
 10. The pressure sensor according to claim 9, whereina silicon oxide insulating layer is formed on the top, the bottom andalong the sides of said piezoresistive sensing element.
 11. The pressuresensor according to claim 9, wherein said device layer is formed on a{110} crystallographic plane of p-type silicon; said piezoresistivesensing elements are formed on said p-type silicon of said device layer;one set of said piezoresistive sensing elements is oriented along a<100> crystallographic direction, and the other set of saidpiezoresistive sensing elements is oriented along a <110>crystallographic direction.
 12. The pressure sensor according to claim9, wherein said device layer is formed on a {110} crystallographic planeof n-type silicon; said piezoresistive sensing elements are formed onsaid n-type silicon of said device layer; one set of said piezoresistivesensing elements is oriented along a <100> crystallographic direction,and the other set of said piezoresistive sensing elements is orientedalong a <110> crystallographic direction.
 13. The pressure sensoraccording to claim 9, wherein said substrate is a cuboid; said handlelayer is formed on a {110} crystallographic plane of single crystallinesilicon; and the corresponding edges of said handle layer and saiddevice layer are oriented along the same crystallographic direction. 14.The pressure sensor according to claim 9, wherein said cap is a cuboid;said cap is formed on a {110} crystallographic plane of singlecrystalline silicon; and the corresponding edges of said cap and saiddevice layer are oriented along the same crystallographic direction. 15.The pressure sensor according to claim 1, wherein said chamber is anenclosure within a metal housing.
 16. The pressure sensor according toclaim 15, wherein a metal diaphragm is further provided; said metaldiaphragm seals said electrically insulating fluid and said pressuresensor die in said chamber; and external pressure to be measured istransmitted from said metal diaphragm to said pressure sensor die. 17.The pressure sensor according to claim 7, wherein said cap and saidsubstrate are both cuboids; said cap is formed on a {110}crystallographic plane of single crystalline silicon; and on the bondedplane between said cap and said substrate, the corresponding edges ofsaid cap and said substrate are oriented along the same crystallographicdirection.